Mika McKinnon

Back in 2012, the Solar Dynamic Observatory recorded one of our star's many burps, but this one was different. This one was symmetrical, and an optical illusion, and spawned stories of an alien ship four times the size of the Earth refuelling.

What you're looking at isn't a solid sphere (or ovoid), it's an illusion created by the combination of just the right viewing angle paired with just the right wavelength filter to make for a dramatic image. You're actually seeing a phenomena known as a coronal cavity.

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If the long, streaking prominences of solar material are the filament of a light bulb, cavities are the empty void around them giving them structure, and streamers are the flow of material up and around the cavity, highlighting the shape. The cavity in this particular image sequence is at just the right angle to look down the barrel of a croissant-shaped void, held in shape by a slinky-spiral of magnetic fields.

Aside from being pretty, cavities are also a hot-topic for solar research because we think they might be the precursors to Coronal Mass Ejections (CMEs). Coronal Mass Ejections are the massive clouds of material that erupt out into the solar system, and, if they intersect the Earth, interfere with satellites or radio communication.

If you want to learn more about the particular cavity that set off the alien space-ship rants, you'll need to go on a blast to the past: When the image was first released, heliophysicist Alex Young produced a video explaining the phenomena, but people still freaked about alien space ships coming to viciously steal gas and... I don't know what the fear is now, two years later, but do something sinister. Shortly afterwards, Mick West wrote a lovely explanation of coronal cavities with a lot of links to other examples, and engaged in a lengthly discussion in the subsequent thread of loose ends and misunderstandings.

Photography credit: NASA/SDO. For more space-weirdness that aren't aliens, check out these strange honeycomb structures on Mars. You can use this form to check out the same cavity in different wavelengths — just look up March 11th and 12th, 2012.